class Solution {
    int[] dx={0,0,1,-1};
        int[] dy={1,-1,0,0};
        int m,n;
    public void solve(char[][] board) {
        m= board.length;
        n=board[0].length;
        for(int j=0;j<n;j++){
            if(board[0][j]=='O') bfs(board,0,j);
            if(board[m-1][j]=='O') bfs(board,m-1,j);
        }
        for(int i=0;i<m;i++){
            if(board[i][0]=='O') bfs(board,i,0);
            if(board[i][n-1]=='O') bfs(board,i,n-1);
        }
        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++)
                if(board[i][j]=='.') board[i][j]='O';
                else if(board[i][j]=='O') board[i][j]='X';
    }
    public void bfs(char[][] board,int i,int j){
        Queue<int[]> q=new LinkedList<>();
        q.add(new int[]{i,j});
        board[i][j]='.';
        while(!q.isEmpty()){
            int[] t=q.poll();
            int a=t[0];
            int b=t[1];
            for(int k=0;k<4;k++){
                int x=a+dx[k];
                int y=b+dy[k];
                if(x>=0&&x<m&&y>=0&&y<n&&board[x][y]=='O'){
                    board[x][y]='.';
                    q.add(new int[]{x,y});
                }
            }
        }
    }
}